Non Destructive Testing Monitoring Thermal Power Stations

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Berenice Blankenship
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1 Non Destructive Testing Monitoring Thermal Power Stations S. Bhowmick Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, India Abstract Shortage of power supply is an issue of great concern in India. In recent years, decommissioning of many dependable units at the Thermal Power Stations has made this problem more acute. Among the major contributing factors, problem of Boiler Tube Failure deserves special mention. However, Condition Monitoring and Maintenance operations at the Thermal Power Stations have played vital roles to resist against this national disaster in power supply. Various advanced methods of Non Destructive Testing like Ultrasonic Inspection for Wall Thickness Measurement and Scale Detection, Eddy Current techniques for crack detection, Remote Visual Inspection at the Turbine sections during short term overhauling have proved to be technical remedial measures to prevent capital overhauling and thereby reducing forced outage at Thermal Power Stations. Every year capital overhauling i.e. complete shutdown of the Boiler Units adversely affects the state revenue. This paper highlights mainly upon the modernization of popular NDT Examinations viz., Wall Thickness Measurement & Crack Detection by Ultrasonic Techniques, Eddy Current Testing, Remote Visual Inspection, XRD Technique for analysis of Boiler Tube scale for research purposes and their applications at the Thermal Power Stations during short term or capital overhauling. Key words: - Non Destructive Testing, Thermal Power Stations, Maintenance Operations. 1. INTRODUCTION Innovations of the conventional methods of day to day methods of maintenance activities have resulted in the saving of both time and labour. But Thermal Power Stations in India are suffering from certain long term problems whose remedies are still in the future prospects of R&D activities. The challenges faced during the day to day operation and maintenance activities at the Thermal Power Stations affects adversely the National Economy and thereby endangering the country s population into a dark future of Power Failure. However, the immediate technical remedy to this problem of Boiler Tube Failure has emphasized the concept of Condition Monitoring activities. Again, very specifically stating any noninvasive techniques utilized to determine the integrity of a material, component or structure or quantitatively measure some characteristic of an object i.e. measure without doing any harm can be considered as Non Destructive Examination. It is definitely for this reason Non Destructive Testing has occupied a vast region both in the industrial as well as in the R&D sector. Every year Thermal Power Stations under renowned PSU sectors spend several lacs for the Condition monitoring operations among which a handful of operations can be considered as Non Destructive Techniques. The concept of this paper is to highlight the several Non Destructive Testing practices at the Thermal Power Stations and thereby

2 emphasizing upon the professional skills and dedications of innumerable field engineers, technicians and scientists for successful implementation of these techniques during the present period of National Crisis in Power Generation. 1.1 Wall Thickness Measurement and Scale Detection using Ultrasonic Techniques: Scale deposition inside the Boiler Tubes is a region of perennial worry at the Power Stations. A handful of Boiler Tube Failure cases can be attributed to Scale Deposition. The scales are mostly composed of Iron oxides and in some cases CaCO3 and certain other compounds depending upon chemical reactions and the nature of water. Purification of river water by using some chemical compounds is a burning issue of research interest in Analytical Chemistry but Scale detection and Wall thickness measurement using Ultrasonic Techniques has proved to be an immediate remedial measure to this long term problem. A tube with scale deposition of more than microns can be considered as failed tube. Accurate scale detection and Wall Thickness Measurement not only predicts the tube condition but also guides the Acid Cleaning procedure so that the tube can be saved before being discarded. 1.1a Basic Principal of Ultrasonic Inspection: Although theoretically any frequency about 20,000 cycles per second (cps) (20 khz) can be thought of as an ultrasonic frequency but in practice ultrasonic transducers and equipment operate in the frequency range of <50 khz to 200MHz. But ultrasonic waves are generated as waves of particle vibrations. The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical pulses are the basis of ultrasonic examination (1). Among the most popularly used non contact Transducers EMAT (Electro Magnetic Acoustic Transducers) deserves special mention. It has a permanent magnet for creating magnetic force and thereby strengthening the wave transmission problem. However for application at the Thermal Power Stations, these transducers have certain operational advantages:- 1) At the power stations, capital overhauling has to be hastened in order to resume power generation as soon as possible. Under these circumstances, the efforts are to get the most accurate thickness measurements at a minimum time. Again, all the tubes within the boiler units have depositions upon their contact surfaces resulting partly from the coal ash and from the impurities present in the steam. Non Contact transducers can easily measure the tube thickness since the outer tube surfaces require no pre cleaning operations resulting in saving of time. 2) Again, it overcomes the limitations of conventional UTs of attenuation in presence of air and as a result no couplant application is requires for its operation. In Pulse Echo Technique (mainly popular at Thermal Power Stations), short bursts of Ultrasonic energy are introduced into the specimen at regular intervals of time. If the pulses encounter a reflecting surface, some or all of the ultrasound energy is reflected. Both the reflected energy and the time delay between transmission of the initial pulse and the reception of any response received from the test piece are measured. A Basic Ultrasonic Instrument

Consists of an electronic signal generating device that produce the alternating voltage, a transducer or probe that emits a beam of Ultrasonic Wave when alternating voltage is applied to it and

3 Consists of an electronic signal generating device that produce the alternating voltage, a transducer or probe that emits a beam of Ultrasonic Wave when alternating voltage is applied to it and recieves the corresponding waves from the test piece, a couplant to act as a medium between the test piece and the transducer, An electronic device to amplify, demodul;ate or modify the signal from the transducer, a dispaly device to demonstrate the trhickness reading, An electronic clock or timer to control the sequence of operations. In this case the Ultrasonic Unit calculates the distance from the known Time and Velocity of Sound Wave. The Velocity of sound differs in different metals and has to be set by the operator. In certain cases, the instrument itself sets it. The total distance travelled by the ultrasonic pulses, is obtained by the distance from the initial pulse or front surface to the back reflection from the back surface. Fig.1.1 A typical Ultrasonic Thickness Measuring Instrument 1.2 Areas of Operation at the Thermal Power Stations: Basically, a Thermal Power Station consists of a furnace to burn the fuel, Steam generator or boiler containing water. Heat generated in the furnace is utilized to convert water in steam, Main power unit such as an engine or turbine to use the heat energy of steam and perform work and Piping system to convey steam and water. In addition to the above equipment the plant requires various auxiliaries and accessories depending upon the availability of water, fuel and the service for which the plant is intended. Periodically, boiler Units are shut down for capital overhauling, the vital time for NDT operations. Operations for Wall Thickness Measurement and scale detection are conducted on various boiler accessories like super heater, economiser, pendant re heater and other boiler accessories which remain in contact with steam. 1.3 Basic Principle of Ultrasonic Flaw Detection: During the process short sound pulses are transmitted into the test piece in order to measure the sound pulse s time of flight from the probe to the reflector and back. The time measurement starts with the electrical transmission pulse, the first pulse. This a short electrical discharge trigering the sound pulse at the transducer crystal. This pulse then travels through the test material and is reflected by a discontinuity or the opposing wall and then returns to the transducer. The recieved oscillations are then converted into electrical pulse and thereby stopping the time measurement. The distance to the reflector can be easily calculated using a simple mathematical relation i.e. S=Ct/2

Where S= Sound path; C= Velocity of sound and t=time of Flight. Now this Time of Flight is graphically displayed on the Ultrasonic Flaw Detector Instrument.

4 Where S= Sound path; C= Velocity of sound and t=time of Flight. Now this Time of Flight is graphically displayed on the Ultrasonic Flaw Detector Instrument. In order to evaluate Visual signals on the screen, there is grid on inside of the CRT. A latest development in the Ultrasonic Flaw Detection technology is the utilisation of Phased Array Instrument. Certain Useful Features vital for power sectors:- Fig.1.2 A Typical Phased Array Instrument 1) Once on site, the angle focus & active area of the array s controlled beam allow it to cover a large area & eliminate the need to connect & reconnect probes to achieve a full image, thereby saving time. 2) Light weight makes it ideal for remote locations & its full color real time sector displays a selectable A-scan for instant & accurate evaluation. 3) Field operation includes detection of subsurface defects, cracks & determination of sizes in pipelines, tubes & weld inspections. This type of instrument can perform both conventional UT and also Phased Array Technique. Angle beam probes are mainly utilized for scanning of the specimen surface by transmitting the sound waves at various angles. Mainly, the subsurface cracks are detected by these probes. The exact location of a flaw can be easily identified by a simple relation between scanning area and probe geometry. 1.4 Areas of Applications at Thermal Power Stations: Huge areas in the turbine section can be counted under its applications. Thus condition monitoring of parts of turbines of varied capacities can be performed during short time overhauling. However the parts that can be subjected to testing are HP/IP Rotor including blades, LP Rotor blades including shroud cover, Shroud band cover of HP & IP cover blades, LP Tie Rod, TG Bearing, Nozzle Plate, HP inner casing (Top & Bottom), Generator rotor retaining ring, Hydrogen seal rings, Fan blades and Control Valve stems & seat etc.

Then, as the coil is brought in close proximity to an electrically conductive material, the alternating magnetic field penetrates the material and generates circular eddy currents.

5 2. Eddy Current Examination Aiding Thermal Power Stations 2.1 Basic Principle of Operation: When an alternating current travels through a coil, an alternating magnetic field is produced and magnetic lines of flux are concentrated at the center of the coil. Then, as the coil is brought in close proximity to an electrically conductive material, the alternating magnetic field penetrates the material and generates circular eddy currents. A crack in the test material obstructs the eddy current flow, lengthens the eddy current path, reduces the secondary magnetic field, and increases the coil Impedance. This change can be detected, amplified, and displayed with help of a portable eddy current testing instrument. The basic principle is to create magnetic fields that generate eddy current flow which is obstructed by a crack in the testing specimen. Fig.2.1 An Example of a Portable Eddy Current Instrument 2.2 Areas of operation At Thermal Power Stations: All the regions where surface cracks are likely to occur can be subjected to eddy current testing. It covers the various parts of turbines previously mentioned, where frequently crack detection is carried on at regular intervals during overhauling. Besides this, it also covers the testing at condenser tubes in the boiler section. Hence, tubes at various sections of boiler like economizer, platen superheater etc. can be subjected to eddy current testing. The conventional methods of Magnetic Particle Inspection and Die Penetrant Inspections are also prevalent at Thermal Power stations, but they require efficient operators for their conduction. Besides, some surface preparations like pre surface cleaning, application of dyes are also required for Die Penetrant Inspection and to some extent in magnetic inspection also. But the modern day Eddy Current instruments are handy and can be used by field engineers at power stations also. Fig.2.2 Various sections of application in the Turbines and Boiler Units

3. Remote Visual Inspection: 3.1 Theoretical Survey & Basic Principle: Detection of any crack or surface failure can be considered as Visual Inspection.

Even the facts recorded by these instruments can be further utitilised for the purpose of R&D analysis. At Thermal Power Stations Visual Inspections are mostly performed manually.

1 A Typical Remote Visual Inspection Camera Certain useful and user friendly features of this instrument:- 1) Its operating temperature of -4 degreef to 115 degreef & storage temperature of -

6 3. Remote Visual Inspection: 3.1 Theoretical Survey & Basic Principle: Detection of any crack or surface failure can be considered as Visual Inspection. In Industrial applications, there are certain regions that can t be accessed through the naked eye and there Remote Visual Inspection methods prove to a promising solution. Even the facts recorded by these instruments can be further utitilised for the purpose of R&D analysis. At Thermal Power Stations Visual Inspections are mostly performed manually. But however, in recent years with prosper of technology certain advanced methods are used. Fig.3.1 A Typical Remote Visual Inspection Camera Certain useful and user friendly features of this instrument:- 1) Its operating temperature of -4 degreef to 115 degreef & storage temperature of - 13degreeF to 140 degreef make this fully compatible for utilization at thermal power stations. 2) All Way articulation of the camera fitted at the top of its probe makes it possible to reach at those places which cannot be accessed by the naked eyes. 3.2 Areas of operation At the Power Stations:- Mainly those locations that cannot be reached b the naked eye comes under its areas of operations. This includes the diaphragms of turbines during the short time overhauling, the internal surfaces of tubes of any locations, boiler hadders etc. It provides a sharp magnified image that can detect cracks or any type of surface deposition thereby visualizing surface topography. Rapid rate of inspection as a part of regular maintenance can be frequently conducted by the field engineers without disturbing much the turbine assemblies at regular intervals which will act as a preventive measure against serious damage & thereby saving time and labour, hence reducing the frequency and duration of short term overhauling duration. Fig.3.2 Various Parts of a turbine Section during Short Term Overhauling

4. Vibration Analysis at the Thermal Power Stations: As mentioned, now a day s portable Laser Vibrometers are very popular for Vibration Based Analysis at Thermal Power Stations.

Further these results can be used both for onsite alignment of shaft and bearings as well as for R&D in design sector by using various software like Matlab etc.

7 4. Vibration Analysis at the Thermal Power Stations: As mentioned, now a day s portable Laser Vibrometers are very popular for Vibration Based Analysis at Thermal Power Stations. The advanced vibrometers also provide the graphs on various domains of time, frequency and amplitude. Further these results can be used both for onsite alignment of shaft and bearings as well as for R&D in design sector by using various software like Matlab etc. in order to design machineries to eliminate the vibration related problems. Although various types of techniques are utilized in analysis of vibration signature but the ost advanced is the non contact type Laser vibrometers. Fig.4.1 A Typical Laser guided Portable Vibrometer Certain noteworthy technical Features:- 1) Light weight, ergonomic & rugged design, hermetic housing make this instrument fully portable & user friendly for operations at the power stations. 2) Non contact velocity measurements can be conducted in the frequency range 0 to 22 KHz, from a distance of.2-30m. The working He-Ne Laser is also eye safe. 4.1 Areas of operation at Thermal Power Stations: This non contact type laser operated device is very vital for online condition monitoring operations. Mainly, vibration signatures can obtain from bearing housing of various machineries at the power stations. Again with the vibration analysis of these profiles machine health can be accessed. Again, the working distance upto 30m max. ensures operators safety at various remote locations. Mainly analysis is conducted with help FFT diagrams. Besides, the machine is capable of performing order analysis with help of tachometer, CPB (Constant Percentage Bandwidth) etc. that states its experimental capabilities. However, the equipments that come under its areas of operation are Cooling tower fans, Induced Draft Fans in the boiler section, Forced Draft Fans in the boiler sections, Turbines of any capacity and Feeders & Crushers of coal handling units. Fig.4.2 Various Sections for Laser Application

5.1 Principle of Operation: 5. X-RAY DEFFRACTION SYSTEM (XRD) X-rays are produced due to rapid deceleration of sufficiently electrically charged particles.

8 5.1 Principle of Operation: 5. X-RAY DEFFRACTION SYSTEM (XRD) X-rays are produced due to rapid deceleration of sufficiently electrically charged particles. Electrons are usually used for this purpose while the radiation being produced in an x-ray tube which contains a source of electrons and two metal electrodes. The high voltage created across these electrodes, rapidly draws the electrons to the anode, as a result of which they strike with very high velocity. X-rays are produced at the point of impact and radiate in all directions. If e is the electron charge (4.80 X 10~10esu) and 1) the voltage (in esu)* across the electrodes, then the kinetic energy (in ergs) of *the electrons on impact is given by the equation KE= ev = ½ mv2, where m is the mass of the electron (9.11 X 10~28gm) and v its velocity just before impact (5). The Bragg law states that (1) The incident beam, the normal to the reflecting plane, and the diffracted beam are always coplanar. (2) The angle between the diffracted beam and the transmitted beam is always 20. This is known as the diffraction angle, and it is this angle, rather than 6, which is usually measured experimentally. Experimentally, the Bragg law can be used by using x-rays of known wavelength X and measuring 0, to determine the spacing d of various planes in a crystal: this is structure analysis and thereby assisting in identifying composition. Generally in Thermal Power Stations XRD tests are utilized for determination of material composition mainly required for Acid Cleaning and various other technical reasons. Fig.5.1 A diagram of the inside construction of a Typical XRD Unit Mainly powder method of Examination can be performed in this method. The specimen to be tested is to be converted into powder and placed in the holder. In this method, the increased number of crystals provides better diffraction angles. 5.2 Application at Thermal Power Stations: It is already known that Scale Deposition has a major contribution to Boiler Tube Failure. Hence In order to take remedial steps, composition of the scales is to be obtained in order to conduct Acid Cleaning or to perform further research work in the chemical branch. By knowing the composition of the scale the composition of compound to be used during Acid Cleaning in capital overhauling can be decided. Generally, the scales are composed of

Fe2O3, Fe3O4, CaCo3 etc. Mainly in house Testing is carried out and the tube samples have to be sent from the Power Stations to the concerned testing authority. 6. Conclusions: Fig.5.

9 Fe2O3, Fe3O4, CaCo3 etc. Mainly in house Testing is carried out and the tube samples have to be sent from the Power Stations to the concerned testing authority. 6. Conclusions: Fig.5.2 A Result obtained in an XRD Unit showing scale composition Upgradation of the existing condition monitoring techniques at the power stations has to be carried extensively with modern equipments and thus long term problem analysis activities can be made self sufficient i.e. the reports obtained from the Non Destructive Testing activities can be used further for problem analysis. Besides these, we can also undertake short term operational problem for analysis detected by these operations. 7. Acknowledgement:-. I would also dedicate this paper to the serious and devoted contributions of the field engineers, scientists, research scholars and NDT professionals for performing Condition Monitoring activities at Thermal Power Stations in India throughout the year. I convey my deep respect, honour and gratitude to the following intellectual personalities, industrial and R&D organisations for their whole hearted and precious contributions to field of Condition Monitoring and Non Destructive Testing. a) Prof. S.K. Roychowdhury (Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur) b) Prof. R. Bhattacharya (Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur) c) Prof. M. Sarangi (Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur) d) All respected Chairman, secretary, other delegates and members of ISNT, Kolkata chapter. e) Central Power Research Institute, Bangalore, an autonomous society under Ministry of Power. f) Central Mechanical Engineering Research Institute, a constituent establishment of CSIR. g) All the Thermal Power Station authorities in India for their patronage and sincere efforts for conduction of NDT operations. 8. References:- 1) Paul E. Mix, INTRODUCTION TO NONDESTRUCTIVE TESTING, A Training Guide, Second Edition, 2005, p ) B.D. Cullity, Elements of X-Ray Diffraction, 1956, p.5.

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